A beamsteered transmit phased array antenna allows electronic steering of the antenna beam direction. This type of antenna system includes a number of individual antenna elements spaced in a regular array. The beam direction of the antenna (i.e., pointing direction) is controlled by the relative phases of the signals radiated by the individual antenna elements. As is known, phased arrays may be used to produce highly directional radiation patterns. Furthermore, performance characteristics normally associated with antennas having large areas can be achieved with a phased array antenna having a comparatively smaller area. Conventional transmit phased array antennas utilize two basic architectures: analog beamforming (ABF) and digital beamforming (DBF).
The basic analog beamforming approach found in the prior art is illustrated in FIG. 1. This system comprises a local radio-frequency (RF) oscillator 10 and an associated signal modulator 12 to produce an RF signal expressed in complex form as: EQU S(t)-S.sub.b (t).multidot.C(t) (1)
where S.sub.b (t) is the complex carrier provided by the RF oscillator and given by: EQU C(t)-A.sub.o e.sup.j.omega..sbsp.o.sup.t ( 2)
where S.sub.b (t) is the complex baseband waveform generated by the signal modulator. The signal S(t) is then distributed to n subarrays 14.sub.1 to 14.sub.n by a splitter 16. Each subarray consists of a digitally controlled complex weighting circuit 18, a power amplifier 20, and an antenna element 22. Each complex weighting circuit produces a controlled phase and amplitude shift in its corresponding subarray RF signal. The signal is then amplified by power amplifier 18 and radiated by antenna element 22.
If each complex weight is represented by P.sub.n, then the signals at the output of each weighting circuit may be represented by P.sub.n .multidot.S(t). The far field radiation pattern will depend upon the number and type of antenna elements, the spacing of the array, and the relative phase and magnitude of the excitation currents applied to the various antenna elements. Generally, the electric field (E-field) generated by the entire phased array is of the form: ##EQU1## where k is the wave vector, r.sub.n is the position of the nth element, and F(k) is proportional to the E-field generated by a single element. The sum in (3) is maximized in the direction of k when EQU P.sub.n .alpha.e.sup.jk.tau..sbsp.m
(assuming approximately equal magnitudes for all the P.sub.n). Thus, the phased array can be electronically steered by manipulating the complex weights P.sub.n.
One of the advantages of a phased array is that a number of beams m can be sent from the same aperture. However, to accomplish this, ABF requires the same number m sets of local oscillators, signal modulators, power splitters, and weighting circuits. At the input of each subarray power amplifier, the m beams are combined to produce a single radiation signal out of each antenna element. The various beam signals then combine in phase in m different directions so as to produce an m-beam output. The resultant E-field of the far field signal is given by: ##EQU2## which represents m independent beams in the far field.
In digital beamforming (DBF), the beam pointing information represented by the complex weights and the modulation information are generated digitally. For one beam, the operation of the complex weighting circuit on the modulated RF signal can be represented as the multiplication of a complex modulation function by a complex weighting number. For multiple beams, these m complex products are summed to produce a single complex number for each subarray. This signal may be represented by: ##EQU3## where S.sub.r,m (t) is either S.sub.m (t) or S.sub.b,m (t). One or more digital to analog (D/A) converters are then utilized to produce an analog representation of V.sub.n (t) for each individual antenna element. Thus, only a single set of digitally controlled complex weighting circuits is required thereby eliminating much of the hardware required to generate a similar signal using ABF techniques. The disadvantage of DBF is that a large number of complex multiplications (m.multidot.n) and complex additions (n) must be performed at a rate equal to the modulation rate. This requires the use of a high speed processor which typically consumes a great deal of power.
Two implementations of DBF have been utilized in the prior art: baseband Cartesian DBF and intermediate frequency (IF) DBF. Cartesian DBF uses a linear in-phase and quadrature (I-Q) modulator and two (2) D/A converters for each complex weighting circuit. The IF DBF technique utilizes D/A converters to directly produce the modulated subarray signals at the intermediate frequency. Upconverters are then required to convert these signals to RF signals. Both Cartesian DBF and IF DBF are characterized by complex implementations which require a significant amount of power. These implementations are not cost effective unless a very large number of beams are required.